A flexible circuit (flex circuit) is a type of electrical connection device that can be bent, twisted or wrapped to fit in extremely small spaces. Flex circuits are excellent for designs with fine line traces and high-density circuitry, and are more suited for dynamic applications and vibration conditions than are conventional printed wiring boards. Using flex circuits sometimes requires transferring an electrical signal from one side of the flex circuit to the opposing side of the flex circuit.
One solution to transition signals from one side of a flex to another side of a flex is to dispose vias proximate the location of a high areal density connector scheme, such as a ball grid array (BGA). Prior Art FIG. 1A illustrates a side view of a flex circuit 110 comprising a first surface 240 and a second opposing surface 245. Flex circuit 110 has a transmission line 202 on surface 240 and a transmission line 204 on surface 245. It is appreciated that a conventional flexible circuit can have a plurality of transmission lines on each surface. Transmission lines 202 and 204 represent one of many transmission lines on each surface of the flex circuit 110. A high areal density connector scheme 120 is disposed on surface 240 and is electrically coupled to transmission line 202. Vias 246 are conventionally disposed proximate the high areal density connector scheme 120 to provide for transitioning a signal from transmission line 204 to transmission line 202. A ground plane 206 is disposed between transmission line 202 and transmission line 204 to increase signal integrity in the flex circuit 10.
To manufacture vias 246 in the flex circuit 110, a hole is formed through the flex circuit 110, including the ground plane 206. A deleterious effect of locating vias 246 proximate the high areal density connector scheme 120 is that the ground plane 206 becomes significantly discontinuous, thus causing grounding problems in the flex circuit 11O. Grounding problems occur because a signal must travel along a transmission line a distance 216 without the benefit of a ground plane 206. Prior Art FIG. 1B illustrates a top view of a portion of a ground plane 206 that is significantly discontinuous as a result of vias 246 disposed proximate connector scheme 120. In some cases, the density of the connector scheme will result in overlapping ground relief holes, thus essentially removing the entire ground plane from the area under the BGA. In addition, placing vias proximate the BGA 120 introduces an additional element of discontinuity in the transmission path on the flex circuit 110.
In other prior art approaches, the vias are located throughout the flex circuit (i.e., not coincident connector scheme 120) to avoid the grounding problems associated with locating the vias near the high-density connector scheme. However, locating the vias throughout the flex circuit distributes electrical discontinuity throughout the entire flex circuit. Every discontinuity along a transmission line is a possible reflection, thus spreading discontinuities can possibly corrupt transmission of a signal along the entire flex circuit.